Au Substrate for Manipulating Surface

Dec 1, 2016 - Taeksu Lee , Jung-Sub Wi , Aram Oh , Hee-Kyung Na , JaeJong Lee , Kwangyeol Lee , Tae Geol Lee , Seungjoo Haam. Nanoscale 2018 10 ...
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Hexagonal Boron Nitride/Au Substrate for Manipulating Surface Plasmon and Enhancing Capability of Surface-Enhanced Raman Spectroscopy Gwangwoo Kim,† Minsu Kim,‡ Chohee Hyun,† Seokmo Hong,‡,§ Kyung Yeol Ma,† Hyeon Suk Shin,*,†,‡,∥,§ and Hyunseob Lim*,‡,§ †

Department of Energy Engineering, ‡Department of Chemistry, ∥Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan 44919, Republic of Korea § Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), UNIST-gil 50, Ulsan 44919, Republic of Korea S Supporting Information *

ABSTRACT: We report on an insulating two-dimensional material, hexagonal boron nitride (h-BN), which can be used as an effective wrapping layer for surface-enhanced Raman spectroscopy (SERS) substrates. This material exhibits outstanding characteristics such as its crystallinity, impermeability, and thermal conductance. Improved SERS sensitivity is confirmed for Au substrates wrapped with h-BN, the mechanism of which is investigated via h-BN thickness-dependent experiments combined with theoretical simulations. The investigations reveal that a stronger electromagnetic field can be generated at the narrowed gap of the h-BN surface, which results in higher Raman sensitivity. Moreover, the h-BN-wrapped Au substrate shows extraordinary stability against photothermal and oxidative damages. We also describe its capability to detect specific chemicals that are difficult to analyze using conventional SERS substrates. We believe that this concept of using an h-BN insulating layer to protect metallic or plasmonic materials will be widely used not only in the field of SERS but also in the broader study of plasmonic and optoelectronic devices. KEYWORDS: hexagonal boron nitride, surface-enhanced Raman spectroscopy, gold nanoparticles, hot spot, insulating layer even in the liquid phase with electron microscopy.9,10 Although these examples imply that graphene can be used as a good metallic wrapping material, this is not a valid approach in some applications that require insulating wrapping layers. For this reason, another two-dimensional material with insulating properties, h-BN, can be used as an insulator wrapping layer. For example, h-BN can protect 2D electron-transport layers from the surface damage without interrupting their intrinsic electronic structures.11−13 Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), a technique for achieving extremely sensitive Raman detection using silica-shelled metal nanoparticles, is one of the applications which requires the use of an insulator wrapping layer.14,15 In SHINERS, a thin silica shell prevents

T

wo-dimensional materials such as graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDs) have been intensively studied over the past decade across a wide field of applications.1 In particular, graphene has recently been investigated as a metallic wrapping material, based on the premise that a two-dimensional architecture is appropriate for use as a packing layer that protects interior materials. Owing to the high conductivity, good flexibility, and chemical stability of graphene, graphenewrapped transition metal oxides have led to performance improvements for electrochemical catalyst2−4 and energy storage materials,5,6 as well as graphene-wrapped sulfur particles7,8 for Li−S batteries. In addition, the long-term stability or cyclability was highly improved; that is because graphene layers protect the active surface of metal oxides or inhibit the diffusion of sulfur anions into electrolytes in Li−S batteries. Moreover, this approach is not restricted to solid substances; graphene-encapsulated liquid samples, referred to as graphene liquid cells, allow for atomic-resolution imaging © 2016 American Chemical Society

Received: September 12, 2016 Accepted: December 1, 2016 Published: December 1, 2016 11156

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ACS Nano direct contact between the metal core and analytes, which protects the overall structures from physical and chemical damage. Compared to traditional insulators such as silica or alumina, h-BN has several advantages: a high crystallinity despite being ultrathin, which facilitates pinhole-free coating on metal particles; a high level of impermeability that prevents chemical penetration; and the highest thermal conductivity among insulating materials. Because of layered-crystal structures with van der Waals interaction, h-BN remains highly crystalline even at a thickness of one atom without any dangling bonds or defects, whereas a thin layer of silica or alumina has a polycrystalline or amorphous crystal structure. Moreover, the basal plane thermal conductivity of bulk h-BN (∼500 Wm−1 K−1) is known to be comparable to that of graphene and is 2 orders of magnitude higher than the corresponding values of silica and alumina.16−18 Accordingly, the high thermal conductivity of h-BN would improve heat dissipation during laser irradiation, which would, in turn, reduce the amount of thermal damage delivered to the analytes. In this study, we investigate the use of Raman spectroscopy on h-BN-wrapped Au particle substrates to demonstrate its effects upon shell-isolated surface-enhanced Raman spectroscopy (SERS). Previously, a graphene-wrapped Au substrate was studied as a SERS substrate.19 However, the enhancement factor (EF) of the Raman intensity was reduced because the metallic properties of graphene would quench the electromagnetic (EM) field on the surface of Au particles (although the stability of the substrate was improved). By way of contrast, an improved EF is observed for the h-BN-wrapped sample, which can be understood in terms of the increased number of analytes adsorbed on the h-BN surface.20,21 However, our systematic study of the thickness-dependent effect of h-BN on SERS, combined with theoretical simulations, suggests a detailed mechanism in which the formation of a narrower nanogap (referred to as a “hotter spot”) induces remarkable improvements in the EF. The dielectric function of h-BN can offset the EM field generated on the metal surface to the h-BN surface, which is suggested as a main origin forming hot spots on the h-BN surface. Furthermore, we study the long-term stability (under laser radiation) of the h-BN-wrapped metal particle substrate and its resistance against chemical damage, which are still important challenges for application of SERS. We also discuss one of the fundamental issues in the current SERS studies, photocatalytic conversion of 4-ABT to the dimeric form on the Au surface, where we showed the difference of fundamental reaction mechanisms on surfaces of Au NPs and h-BN-wrapped Au NPs.

Scheme 1. Schematic Illustration of Shell-Isolated SERS Process on an h-BN/Au/SiO2 Substrate

Figure 1. SERS measurement using an h-BN/Au/SiO2 substrate. (a) AFM image of CVD-grown h-BN on a Au/SiO2 substrate. (Inset) Height profile of the white dotted line in panel a. (b) SERS measurement of rhodamine 6G (R6G) on bare SiO2, h-BN/SiO2, Au/SiO2, and 7 nm h-BN/Au/SiO2.

average gap length between Au surfaces was ∼12.4 nm (Figure S1b,c). We performed cross-sectional high-resolution transmission electron microscopy (HR-TEM) in order to reveal the real structure of h-BN/Au/SiO2 substrate. As shown in Figure S2, the Au NPs were well-wrapped with 4−5 layers of h-BN film, which indicated a direct evidence for the hat-shaped structure. Rhodamine 6G (R6G), the most commonly used SERS molecule,24,25 was selected as an analyte. The bare SiO2, h-BN/SiO2, Au/SiO2, and h-BN/Au/SiO2 were immersed in the 1 mM R6G aqueous solution for 30 min and were then rinsed with deionized (DI) water several times. Figure 1b shows the Raman spectra of R6G measured on each substrate with a 532 nm excitation laser, which falls in the surface plasmon resonance (SPR) range of Au/SiO2 and the hBN/Au/SiO2 substrate (Figure S3). While unnoticeable Raman peaks are shown on bare SiO2 (black), SERS effects are confirmed both on Au/SiO2 and h-BN/Au/SiO2. Interestingly, the Raman signal on h-BN/Au/SiO2 (red) is ∼3.5 times stronger than that on Au/SiO2 (blue), which is inconsistent with the typical behavior in which the insulating coating on metal particles reduces the Raman enhancement.14,15 In addition to 532 nm excitation laser, we confirmed the improved Raman signal of R6G on h-BN/Au/SiO2 substrate by using a 633 nm laser (Figure S4). The SERS measurement of R6G with different concentration was also performed on Au/SiO2 and hBN/Au/SiO2 substrate (Figure S5). We observed the enhanced signal of R6G even at 10−9 M concentration but no signal of 10−12 M concentration. In order to elucidate this mechanism in detail, we investigated the Raman enhancement as a function of the hBN thickness. By repeating the transfer of the CVD-grown monolayer h-BN sheets,22 samples with 1 to 10 layers of h-BN were prepared, and the ME or CVD-grown multilayer h-BN sheets23 were used for preparing samples with thicker h-BN

RESULTS AND DISCUSSION The h-BN-wrapped Au particle substrate (h-BN/Au/SiO2) was first prepared by transferring either mechanically exfoliated (ME) h-BN or chemical vapor deposition (CVD)-grown h-BN sheets22,23 onto the Au particles on the SiO2 substrate (Au/ SiO2), which was prepared by the thermal evaporation of a 10 nm Au film. The as-prepared samples were annealed at 400 °C with Ar flow (100 sccm) in a vacuum tube furnace to make better contact by removing trapped solvent or gas molecules at the interface between the h-BN and Au particles (Scheme 1). The surface morphology of h-BN-wrapped Au/SiO2 substrates (h-BN/Au/SiO2) after annealing was characterized by atomic force microscopy (AFM) (Figure 1a) and scanning electron microscopy (SEM) (Figure S1a). The mean diameter of Au nanoparticles after the whole process was ∼27.7 nm, and the 11157

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ACS Nano films. Figure 2a shows the Raman spectra of R6G for various hBN thicknesses. The representative Raman mapping images

then enters a gradually decreasing region (region II: 7 to 30 nm). The thickness dependence confirmed in this study implies that the higher sensitivity of h-BN/Au/SiO2 cannot be mainly attributed to the mechanism explained above, that is, the increased number of detectable molecules on h-BN.20,21 Although the surface adsorption properties of h-BN are known to depend on the thickness, the degree of surface adsorption on atomically thin h-BN is expected to be higher than that of thicker h-BN because of the larger conformational change and highly wrinkled architecture.20,21,26 However, a ∼1.3 times stronger Raman signal is only measured on onelayer (1 L) h-BN/Au/SiO2, but a much stronger EF is measured on the thicker h-BN. This inconsistency indicates that another mechanism is required to explain the thickness dependence (Figure 2c), and the anomalous bell-shaped distribution generally indicates that the two parameters depending on h-BN thickness simultaneously influence the Raman enhancement in an opposite direction. In order to understand the origin of anomalous behavior in Raman enhancement by the h-BN wrapping layer, finitedifference time-domain (FDTD) simulations were performed to investigate the EM field generation on h-BN/Au/SiO2 under laser irradiation. Two important relationships were demonstrated: (i) a longer distance from the Au surface to the h-BN surface reduces the EM field; (ii) a narrower nanogap at the hBN surface induces a stronger overlapping of the EM field if the h-BN thickness lies in region I (Figure 2c). The first FDTD simulation model consisted of a single 30 nm Au particle wrapped with hat-shaped h-BN layers of different thicknesses (Figure S6). In these models, the strength of the EM field on the h-BN surface gradually decreases as the h-BN thickness increases (Figure S6b,c). The trend in the simulation results is similar to the simulation results in the literature concerning SHINERS effects14,15 but does not match our experimental trend. Nevertheless, these results reveal the salient point. Since h-BN has a permittivity (εr = 4.5) higher than that in air (εr =

Figure 2. h-BN thickness-dependent SERS measurement of R6G on a h-BN/Au/SiO2 substrate. (a) SERS spectra of R6G on CVD h-BN (0−5 layers) and ME h-BN (7 and 20 nm). SERS mapping images of the R6G 613 cm−1 peak on (b) CVD h-BN (5 layers) and ME hBN (20 nm). (c) SERS enhancement of the R6G 613 cm−1 peak with increasing h-BN thickness. The marked blue circles and red triangles are the EFs on CVD-grown h-BN and ME h-BN, respectively.

shown in Figure 2b (CVD h-BN and ME h-BN) demonstrate the homogeneous and stronger Raman enhancement on the hBN-wrapped region. The relative EFs of the Raman peak at 613 cm−1 (Ih‑BN/Au/SiO2/IAu/SiO2) as a function of h-BN thickness (Figure 2c) reveal a bell-shaped distribution; the EF reaches its maximum for an h-BN thickness of ∼7 nm. (It is worth noting that all EF values in this work are based on the intensity of the peak at 613 cm−1.) More specifically, the peak intensity of R6G increases until a thickness of ∼7 nm (region I: 0 to 7 nm) and

Figure 3. Three-dimensional FDTD simulation for revealing the hot spot effect on the h-BN/Au/SiO2 substrate. (a) Scheme of the h-BN/Au/ SiO2 structure (t, h-BN thickness; w, distance between surfaces of Au nanoparticles; d, distance between centers of Au nanoparticles). (b−d) Side-view and top-view simulation images (left and right columns, respectively) of EM field distribution on the h-BN (0, 5, and 10 nm)/Au/ SiO2 substrate (diameter of Au = 30 nm, d = 40 nm, laser wavelength = 532 nm, incident E-field = 1 V/m). The position of the top-view simulation is marked by the red line in the side-view image. The h-BN film is marked by the white dotted line in the side-view image. (e) Plot of maximum EM field and h-BN thickness in the h-BN/Au/SiO2 substrate with different d values (35, 40, and 45 nm). 11158

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Figure 4. Photothermal and chemical stability of 3L h-BN/Au/SiO2 substrate. SERS spectra of R6G on the Au/SiO2 substrate (a) with and (b) without h-BN protection at different time points (laser power = 0.1 mW, time interval = 15 min). SERS spectra of R6G on (c) 3L h-BN/Au/ SiO2 and (d) Au/SiO2 substrate before and after being treated by HNO3 for 30 min.

fields depends on the wrapping extent because the valley point of the h-BN layer is changed. The h-BN thickness showing the maximum EM field was 5 and 10 nm for 70 and 30% wrapped Au NPs in h-BN/Au/SiO2 with a 10 nm gap of Au NPs. These are similar to the simulation result with 50% wrapped Au NPs in the h-BN/Au/SiO2 substrate. We also simulated the EM field with graphene as a wrapping layer; the modulated EM field did not occur because the EM field was almost quenched by the metallic property of graphene (Figure S10). We also tested the potential use of the h-BN layer as a heat sink or a protective wrapping layer. First, the time-dependent Raman measurements with R6G were carried out on both Au/ SiO2 and 3L h-BN/Au/SiO2 in order to observe any photoinduced thermal damage, which is a well-known side effect of SERS.19,28 During a 450 min measurement under 0.1 mW laser irradiation, the Raman signal of R6G remained steady on 3L h-BN/Au/SiO2 (92.7% after 450 min, Figure 4a), whereas the Raman intensity decreased quickly on the Au/SiO2 sample (33.6% after 450 min, Figure 4b). We believe that the outstanding stability against photothermal damage of 3L h-BN/ Au/SiO2 is caused by the ultrafast heat dissipation through the h-BN layer, which is a result of the excellent thermal conductivity of h-BN.16,18 The chemical stability of h-BN/Au/SiO2 against surface oxidation was also investigated. Both Au/SiO2 and 3L h-BN/ Au/SiO2 substrates were immersed in a 15% nitric acid solution for 30 min and then were washed with DI water several times; R6G molecules were adsorbed on the acid-treated substrates with the same process. The Raman spectra measured before and after the acid treatment are displayed in Figure 4c,d. The negligible change of Raman intensity on 3L h-BN/Au/SiO2 (91.5%) even after strong acid treatment indicates that Au particles are protected from the acid owing to the high impermeability of the h-BN layer. In contrast, the large Raman signal decrease of Au/SiO2 (30.2%) is indicative of surface damage to the Au particles as a result of the acid treatment. In case of thicker h-BN film (7 nm), the good stability against photothermal damage and surface oxidation was also confirmed, as shown in Figure S11. This resistance against

1), the EM field on the h-BN surface (B in Figure S7) is stronger than the EF field at position A in Figure S7, the same position from the bare Au surface.27 Consequently, the maximum EF position can be translated from the core Au surface to the h-BN surface, which may facilitate the strong overlapping of EF fields at certain “hot spots” if the distance between Au particles is fixed. To check this capability, additional simulations were performed for the case of two Au particles (30 nm) (Figure 3a) and a double-hat-shaped h-BN layer (d, interparticle distance; w, gap distance; t, thickness of h-BN). Figure 3b−d shows the simulation results with h-BN thicknesses of 0, 5, and 10 nm when d = 40 nm. The results associated with these conditions are the closest to the experimentally confirmed values (Figure 2). In these models, circular h-BN surfaces wrapping each Au particle are merged together, forming a valley-like architecture, and when

t2 +

2

( d2 )

=

(d − w) 2

+ t,

approximately, t is slightly larger than t = w/2 (Figure S8). Before the h-BN surfaces are merged (region I in Figure 2c), the distance between h-BN surfaces becomes closer as the hBN thickness increases, which induces the stronger overlapping of EM fields, resulting in a stronger EM field (i.e., the hotter spot, marked with white arrows in Figure 3c,d) at the h-BN surface. However, if the h-BN is thicker than the starting t value of merge, the strength of the EM field becomes weaker because the merged position becomes far from the core Au surface (region II in Figure 2c). The EM strength as a function of the h-BN thickness is plotted in Figure 3e for the cases of d = 35, 40, and 45 nm. The simulation trend for d = 40 nm (blue) and the experimental trend in Figure 2c match well, indicating that Raman enhancement from using h-BN operates according to the second proposed mechanism (according to which the EM field modulation by h-BN contributes the high sensitivity of h-BN). In order to investigate the effect of the wrapping extent of Au NPs with h-BN, we carried out FDTD simulations for h-BN/ Au substrates with 30 and 70% wrapped Au NPs with h-BN layers (Figure S9). The h-BN thickness showing maximum EM 11159

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Figure 5. SERS of analytes that are difficult to detect on the SERS substrate. (a) SERS spectra of 4-aminobenzenethiol (4-ABT) on the 3L hBN/Au/SiO2 and Au/SiO2 substrates. (b) Scheme to explain the mechanism. Raman spectrum of dimercaptoazobenzene (DMAB) was observed on the Au substrate because of catalytic reactions. (c) SERS spectra benzo(α)pyrene on h-BN/Au/SiO2 and Au/SiO2 substrates. (d) Schematic mechanism to explain SERS of benzo(α)pyrene on h-BN/Au/SiO2 and Au/SiO2 substrates.

5b). It is worth noting that 1L h-BN is not sufficient for prohibiting the photocatalytic reaction because of defects and imperfect grain boundaries in 1L h-BN (see Figure S13b). Finally, we show that h-BN/Au/SiO2 is also useful for the detection of specific molecules slightly adsorbed on Au NPs, such as polycyclic aromatic hydrocarbon (PAC). Benzo(α)pyrene (B(α)P) consisting of 5-benzene rings was chosen for this study, and Raman spectra of B(α)P were measured on Au/ SiO2 and 3L h-BN/Au/SiO2. Since the weak interaction between PAC molecules and the metal surface prohibits their adsorption on the metal surface, their Raman detection is very difficult using conventional SERS. Indeed, no Raman signal for B(α)P was measured on Au/SiO2, while noticeable and characteristic bands of B(α)P were detected on h-BN/Au/ SiO2 (Figure 5c); that is because the π−π interaction between B(α)P and h-BN enlarges the surface adsorption coverage, and the π−π interaction between B(α)P and h-BN enlarges the surface adsorption coverage on h-BN/Au/SiO2 (Figure 5d).33,34

thermal oxidation was further studied by annealing the samples at 400 °C under ambient conditions. The results illustrate a tendency that is to the case of chemical oxidation (Figure S12). We propose that the outstanding thermal conductivity and impermeability of the h-BN layer allow h-BN/Au/SiO2 to function as an ultrastable SERS substrate, protecting both analytes and metal particles from both photothermal and chemical damage. In addition to the function of h-BN as a protective barrier, this material can deter undesired photocatalytic reactions on metal particles. 4-Aminobenzenethiol (4-ABT) is a historically important molecule for SERS, which has often been used for investigating the charge-transfer-driven SERS mechanism,29,30 but recent studies revealed that 4-ABT can be transformed into dimercaptoazobenzene (DMAB) on metal nanoparticles under illumination of light in the presence of O2 molecules31,32 (see Figure S13a). We performed the SERS experiments with 4ABT on 3L h-BN/Au/SiO2 to confirm that the h-BN layer can prevent this undesired photocatalytic reaction of 4-ABT into DMAB. Figure 5a,b shows Raman spectra obtained from 4ABT/3L h-BN/Au/SiO2 and 4-ABT/Au/SiO2 substrates in air. While strong and characteristic bands of DMAB (marked with blue diamonds) are observed on the Au/SiO2 substrate (blue in Figure 5a), those peaks are not detected on the 3L h-BN/Au/ SiO2 substrate (black in Figure 5a), which implies that the hBN layer can inhibit the dimerization of 4-ABT. Even though the exact mechanism on the oxidation of 4-ABT to DMAB is still not conclusive, Huang suggested that SPR-assisted activation of 3O2 and subsequent formation of Au oxides or hydroxides by the SPR-induced local heating effect may play key roles in the oxidation of 4-ABT to DMAB.31 By considering these mechanisms, we propose that the isolation of O2 molecules from the surface of Au NPs and the rapid heat dissipation by h-BN layers would play important roles to avoid the dimerization of 4-ABT on the 3L h-BN/Au/SiO2 (Figure

CONCLUSION In summary, our results constitute the demonstration of the extraordinary Raman enhancement behavior of h-BN/Au/SiO2. The thickness-dependent experimental and simulation results have demonstrated that the modulated hotter spots on h-BN/ Au/SiO2 can result in higher Raman sensitivity, despite the decay in the EM field owing to the insulating wrapping layer. Furthermore, the outstanding stability of the h-BN layer against photoinduced damage to the analyte or oxidative damage to the Au particles was confirmed. Moreover, we have suggested several useful applications for specific molecules such as 4-ABT and B(α)P, whose analysis is difficult using conventional SERS methods. We anticipate that this approach of using an atomically thin h-BN insulating layer to protect metallic and plasmonic materials will be widely used not only in the field of 11160

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ACKNOWLEDGMENTS The authors are thankful to N. Hayazawa for fruitful discussions. This work was supported by an NRF grant (No. NRF-2014R1A2A2A01007136), the IBS (IBS-R019-D1), and a grant (CASE-2013M3A6A5073173) from the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project.

SERS but also in the broader study of plasmonic and optoelectronic devices.

METHODS SERS Measurement. Monolayers and multilayers of h-BN film were synthesized on Pt foil and Al2O3 substrate using ammonia borane as a CVD precursor. Au particles on the SiO2 substrate (Au/SiO2) were prepared by the thermal evaporation of a 10 nm Au film. The precise experimental details can be found in our previous reports.22,23 The produced h-BN films were transferred from Pt and Al2O3 to Au/ SiO2 substrate using wet-transfer methods (electrochemical delamination and etching of Al2O3 suface). The as-prepared samples (h-BN/ Au/SiO2) were annealed at 400 °C with Ar flow (100 sccm) in a vacuum tube furnace. The surface morphology of the samples was characterized by SEM (Verios 460, FEI) and AFM (Dimension Icon, Bruker). The UV−visible absorption spectrum (Cary 5000 UV−vis− NIR, Agilent) was measured to estimate the absorbance spectra of hBN/Au/quartz substrate. To check the SERS effect, the h-BN/Au/ SiO2 substrate was immersed in the 1 mM R6G aqueous solution, 1 mM 4-ABT ethanol solution, and 1 mM B(α)P acetone solution for 30 min and was then rinsed with DI water several times. Raman spectra were measured using a micro-Raman spectrometer (alpha300, WITec GmbH) with a 532 nm laser. FDTD Simulation. The FDTD simulation was used to demonstrate the EM field distribution in the h-BN/Au/SiO2 substrate, which revealed the contribution of h-BN to EM field enhancement. The diameter of Au nanoparticles was set at 30 nm, and the distance between centers of Au was 35, 40, and 45 nm. The h-BN with permittivity of 4.5 is controlled from 0 to 15 nm thickness.27 We set the wavelength of the incident light to be 532 nm and the waveform to be sinusoidal with an amplitude of 1 V/m.

REFERENCES (1) Ferrari, A. C.; Bonaccorso, F.; Fal’ko, V.; Novoselov, K. S.; Roche, S.; Boggild, P.; Borini, S.; Koppens, F. H. L.; Palermo, V.; Pugno, N.; Garrido, J. A.; Sordan, R.; Bianco, A.; Ballerini, L.; Prato, M.; Lidorikis, E.; Kivioja, J.; Marinelli, C.; Ryhanen, T.; Morpurgo, A.; et al. Science and Technology Roadmap for Graphene, Related TwoDimensional Crystals, and Hybrid Systems. Nanoscale 2015, 7, 4598− 4810. (2) Deng, D.; Novoselov, K. S.; Fu, Q.; Zheng, N.; Tian, Z.; Bao, X. Catalysis with Two-Dimensional Materials and Their Heterostructures. Nat. Nanotechnol. 2016, 11, 218−230. (3) Li, L.; Raji, A. R. O.; Tour, J. M. Graphene-Wrapped MnO2− Graphene Nanoribbons as Anode Materials for High-Performance Lithium Ion Batteries. Adv. Mater. 2013, 25, 6298−6302. (4) Lee, J. S.; You, K. H.; Park, C. B. Highly Photoactive, Low Bandgap TiO2 Nanoparticles Wrapped by Graphene. Adv. Mater. 2012, 24, 1084−1088. (5) Zhou, G.; Wang, D. W.; Li, F.; Zhang, L.; Li, N.; Wu, Z. S.; Wen, L.; Lu, G. Q.; Cheng, H. M. Graphene-Wrapped Fe3O4 Anode Material with Improved Reversible Capacity and Cyclic Stability for Lithium Ion Batteries. Chem. Mater. 2010, 22, 5306−5313. (6) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Li-O2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2012, 11, 19−29. (7) Wang, H.; Yang, Y.; Liang, Y.; Robinson, J. T.; Li, Y.; Jackson, A.; Cui, Y.; Dai, H. Graphene-Wrapped Sulfur Particles as a Rechargeable Lithium−Sulfur Battery Cathode Material with High Capacity and Cycling Stability. Nano Lett. 2011, 11, 2644−2647. (8) Lu, S.; Cheng, Y.; Wu, X.; Liu, J. Significantly Improved LongCycle Stability in High-Rate Li−S Batteries Enabled by Coaxial Graphene Wrapping over Sulfur-Coated Carbon Nanofibers. Nano Lett. 2013, 13, 2485−2489. (9) Yuk, J. M.; Park, J.; Ercius, P.; Kim, K.; Hellebusch, D. J.; Crommie, M. F.; Lee, J. Y.; Zettl, A.; Alivisatos, A. P. High-Resolution EM of Colloidal Nanocrystal Growth Using Graphene Liquid Cells. Science 2012, 336, 61−64. (10) Chen, Q.; Smith, J. M.; Park, J.; Kim, K.; Ho, D.; Rasool, H. I.; Zettl, A.; Alivisatos, A. P. 3D Motion of DNA-Au Nanoconjugates in Graphene Liquid Cell Electron Microscopy. Nano Lett. 2013, 13, 4556−4561. (11) Cui, X.; Lee, G. H.; Kim, Y. D.; Arefe, G.; Huang, P. Y.; Lee, C. H.; Chenet, D. A.; Zhang, X.; Wang, L.; Ye, F.; Pizzocchero, F.; Jessen, B. S.; Watanabe, K.; Taniguchi, T.; Muller, D. A.; Low, T.; Kim, P.; Hone, J. Multi-Terminal Transport Measurements of MoS2 Using a Van Der Waals Heterostructure Device Platform. Nat. Nanotechnol. 2015, 10, 534−540. (12) Britnell, L.; Gorbachev, R. V.; Jalil, R.; Belle, B. D.; Schedin, F.; Mishchenko, A.; Georgiou, T.; Katsnelson, M. I.; Eaves, L.; Morozov, S. V.; Peres, N. M. R.; Leist, J.; Geim, A. K.; Novoselov, K. S.; Ponomarenko, L. A. Field-Effect Tunneling Transistor Based on Vertical Graphene Heterostructures. Science 2012, 335, 947−950. (13) Wang, J. I. J.; Yang, Y.; Chen, Y. A.; Watanabe, K.; Taniguchi, T.; Churchill, H. O.; Jarillo-Herrero, P. Electronic Transport of Encapsulated Graphene and WSe2 Devices Fabricated by Pick-Up of Prepatterned hBN. Nano Lett. 2015, 15, 1898−1903. (14) Li, J. F.; Huang, Y. F.; Ding, Y.; Yang, Z. L.; Li, S. B.; Zhou, X. S.; Fan, F. R.; Zhang, W.; Zhou, Z. Y.; Wu, D. Y.; Ren, B.; Wang, Z. L.; Tian, Z. Q. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nature 2010, 464, 392−395.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06153. SEM image of h-BN/Au/SiO2 after annealing process; cross-sectional HR-TEM images of h-BN/Au/SiO 2 substrate; UV−vis absorbance spectra of h-BN/Au/ quartz and Au/quartz substrates; SERS measurement of R6G on h-BN/Au/SiO2 using 633 nm laser; SERS measurement of R6G with different concentration; 3DFDTD simulation for h-BN-wrapped single Au nanoparticle; change of the strongest EM position from Au surface to h-BN surface; model consisting of two Au particles and a double-hat-shaped h-BN layer; plot of maximum EM field and h-BN thickness in h-BN/Au/ SiO2 substrate with 30 and 70% wrapping structures; comparison of simulated the EM field with h-BN and graphene as a wrapping layer; stability of 7 nm h-BN/ Au/SiO2 against photothermal damage and surface oxidation; thermal stability of h-BN/Au/SiO2 substrate; Raman spectra of 4-ABT on Au/SiO2 depending on the different atmosphere and the number of h-BN layers (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hyeon Suk Shin: 0000-0003-0495-7443 Notes

The authors declare no competing financial interest. 11161

DOI: 10.1021/acsnano.6b06153 ACS Nano 2016, 10, 11156−11162

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ACS Nano (15) Li, J. F.; Anema, J. R.; Wandlowski, T.; Tian, Z. Q. Dielectric Shell Isolated and Graphene Shell Isolated Nanoparticle Enhanced Raman Spectroscopies and Their Applications. Chem. Soc. Rev. 2015, 44, 8399−8409. (16) Wang, C.; Guo, J.; Dong, L.; Aiyiti, A.; Xu, X.; Li, B. Superior Thermal Conductivity in Suspended Bilayer Hexagonal Boron Nitride. Sci. Rep. 2016, 6, 25334. (17) Seol, J. H.; Jo, I.; Moore, A. L.; Lindsay, L.; Aitken, Z. H.; Pettes, M. T.; Li, X.; Yao, Z.; Huang, R.; Broido, D.; Mingo, N.; Ruoff, R. S.; Shi, L. Two-Dimensional Phonon Transport in Supported Graphene. Science 2010, 328, 213−216. (18) Jo, I.; Pettes, M. T.; Kim, J.; Watanabe, K.; Taniguchi, T.; Yao, Z.; Shi, L. Thermal Conductivity and Phonon Transport in Suspended Few-Layer Hexagonal Boron Nitride. Nano Lett. 2013, 13, 550−554. (19) Xu, W.; Xiao, J.; Chen, Y.; Chen, Y.; Ling, X.; Zhang, J. Graphene-Veiled Gold Substrate for Surface-Enhanced Raman Spectroscopy. Adv. Mater. 2013, 25, 928−933. (20) Cai, Q.; Mateti, S.; Yang, W.; Jones, R.; Watanabe, K.; Taniguchi, T.; Huang, S.; Chen, Y.; Li, L. H. Boron Nitride Nanosheets Improve Sensitivity and Reusability of Surface Enhanced Raman Spectroscopy. Angew. Chem., Int. Ed. 2016, 55, 8405−8410. (21) Cai, Q.; Mateti, S.; Watanabe, K.; Taniguchi, T.; Huang, S. M.; Chen, Y.; Li, L. H. Boron Nitride Nanosheet Veiled Gold Nanoparticles for Surface Enhanced Raman Scattering. ACS Appl. Mater. Interfaces 2016, 8, 15630−15636. (22) Kim, G.; Jang, A. R.; Jeong, H. Y.; Lee, Z.; Kang, D. J.; Shin, H. S. Growth of High-Crystalline, Single-Layer Hexagonal Boron Nitride on Recyclable Platinum Foil. Nano Lett. 2013, 13, 1834−1839. (23) Jang, A. R.; Hong, S.; Hyun, C.; Yoon, S. I.; Kim, G.; Jeong, H. Y.; Shin, T. J.; Park, S. O.; Wong, K.; Kwak, S. K.; Park, N.; Yu, K.; Choi, E.; Mishchenko, A.; Withers, F.; Novoselov, K. S.; Lim, H.; Shin, H. S. Wafer-Scale and Wrinkle-Free Epitaxial Growth of SingleOrientated Multilayer Hexagonal Boron Nitride on Sapphire. Nano Lett. 2016, 16, 3360−3366. (24) Hildebrandt, P.; Stockburger, M. Surface-Enhanced Resonance Raman Spectroscopy of Rhodamine 6G Adsorbed on Colloidal Silver. J. Phys. Chem. 1984, 88, 5935−5944. (25) Nie, S.; Emory, S. R. Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering. Science 1997, 275, 1102−1106. (26) Cai, Q.; Li, L. H.; Yu, Y.; Liu, Y.; Huang, S.; Chen, Y.; Watanabe, K.; Taniguchi, T. Boron Nitride Nanosheets as Improved and Reusable Substrates for Gold Nanoparticles Enabled Surface Enhanced Raman Spectroscopy. Phys. Chem. Chem. Phys. 2015, 17, 7761−7766. (27) Gielisse, P. J.; Mitra, S. S.; Plendl, J. N.; Griffis, R. D.; Mansur, L. C.; Marshall, R.; Pascoe, E. A. Lattice Infrared Spectra of Boron Nitride and Boron Monophosphide. Phys. Rev. 1967, 155, 1039−1046. (28) Xu, W.; Ling, X.; Xiao, J.; Dresselhaus, M. S.; Kong, J.; Xu, H.; Liu, Z.; Zhang, J. Surface Enhanced Raman Spectroscopy on a Flat Graphene Surface. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 9281− 9286. (29) Huang, Y. F.; Zhu, H. P.; Liu, G. K.; Wu, D. Y.; Ren, B.; Tian, Z. Q. When the Signal is Not from the Original Molecule To Be Detected: Chemical Transformation of para-Aminothiophenol on Ag during the SERS Measurement. J. Am. Chem. Soc. 2010, 132, 9244− 9246. (30) Huang, Y. F.; Wu, D. Y.; Zhu, H. P.; Zhao, L. B.; Liu, G. K.; Ren, B.; Tian, Z. Q. Surface-enhanced Raman spectroscopic study of paminothiophenol. Phys. Chem. Chem. Phys. 2012, 14, 8485−8497. (31) Huang, Y. F.; Zhang, M.; Zhao, L. B.; Feng, J. M.; Wu, D. Y.; Ren, B.; Tian, Z. Q. Activation of Oxygen on Gold and Silver Nanoparticles Assisted by Surface Plasmon Resonances. Angew. Chem., Int. Ed. 2014, 53, 2353−2357. (32) Xu, P.; Kang, L.; Mack, N. H.; Schanze, K. S.; Han, X.; Wang, H. L. Mechanistic Understanding of Surface Plasmon Assisted Catalysis on a Single Particle: Cyclic Redox of 4-Aminothiophenol. Sci. Rep. 2013, 3, 2997.

(33) Guerrini, L.; Garcia-Ramos, J. V.; Domingo, C.; Sanchez-Cortes, S. Sensing Polycyclic Aromatic Hydrocarbons with DithiocarbamateFunctionalized Ag Nanoparticles by Surface-Enhanced Raman Scattering. Anal. Chem. 2009, 81, 953−960. (34) Liu, X.; Wang, J.; Wu, Y.; Fan, T.; Xu, Y.; Tang, L.; Ying, Y. Compact Shielding of Graphene Monolayer Leads To Extraordinary SERS-Active Substrate with Large-Area Uniformity and Long-Term Stability. Sci. Rep. 2015, 5, 17167.

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DOI: 10.1021/acsnano.6b06153 ACS Nano 2016, 10, 11156−11162